1. Field of the Invention
The present invention relates to a spectral optical element that is used in a spectral analytical instrument and an optical apparatus using a plurality of wavelengths, and particularly, the present invention relates to a spectral optical element using a diffraction optical element.
2. Description of the Related Art
A diffraction optical element represented by a diffraction grating has been widely used in a spectral analytical instrument for analysis of an optical spectrum. In the spectral analysis, a high-energy usability over a broad band is required. In order to obtain high diffraction efficiency in the broad band, a reflection diffraction grating is suitable. In addition, the reflection diffraction grating has been widely used in the spectral analytical instrument because a change ratio of a diffraction angle with respect to a wavelength, namely, a wavelength angle dispersion property is favorable.
The reflection diffraction grating, in which a grating interval of the diffraction grating is about a wavelength, has a large wavelength dispersion and has a broad band and a high efficiency for a TM (transverse magnetic) mode, however, this involves a problem that the efficiency thereof is low for a TE (transverse electric) mode and a wavelength loss property does not become stable. Such a diffraction grating indicates a complex wavelength loss property because it is operated in a resonance region so that it cannot realize stable operation over a broad band.
On the contrary, a diffraction grating, in which the grating interval is ten times and over the wavelength, uses a high-order diffracted light in a so-called Littrow mounting or near arrangement, and this makes it possible to realize a high efficiency in a high wavelength dispersion and to lower a polarization dependent loss (PDL) (for example, refer to Hiroshi Kubota, “Wave Optics”, Iwanami Shoten, 1971, chapter ten) However, an available band pass is restricted by a free spectrum range, and generally, it is not more than 100 nm. In such a diffraction grating, diffracted light of many orders are generated, so that diffracted light of unnecessary orders enter with predetermined output angle from a relation between wavelength, incident and output angles, and a grating constant. Therefore, the available band pass is limited.
On the other hand, the transmission diffraction grating has been also widely used depending on a purpose. The transmission diffraction grating is preferable to realize a low PDL and high diffraction efficiency in a specific wavelength.
Although the transmission diffraction grating can realize both a low PDL and high diffraction efficiency in a specific wavelength band, the band pass is not broad. In addition, in light of a wavelength λ and a λ/m (m is a natural number not less than 2), diffracted light of a diffraction order of several m having the same diffraction angles are generated, and thereby, its band pass is limited.